1. Field of the Invention
This invention relates to a fuel cell system, and more particularly to an improvement of efficiency in using a full gas.
2. Description of Related Art
There is known a fuel cell system loaded on a fuel cell automobile and the like. The fuel cell system has a fuel cell stack, as a main body, which is constructed by an anode (i.e., a hydrogen electrode or a fuel electrode) and a cathode (i.e., an oxygen electrode). The fuel cell system also has a tank which stores fuel gas such as hydrogen supplied to the anode, a pump which returns exhaust gas including unused fuel gas to the original anode and the like. In the fuel cell stack, the hydrogen and the oxygen included in the air react on each other to generate an electric power.
In the fuel cell system, as a cell reaction proceeds, nitrogen in oxidized gas as well as water and moisture for humidification pass through an electrolyte membrane from the cathode, and exude to the side of the anode. Therefore, partial pressure of the nitrogen and the moisture (hereinafter, referred to as “impurities”) increases, and concentration of unused fuel gas in the exhaust gas decreases. Thereby, a power generation ability of the fuel cell stack decreases.
In this view, there is known such a technique that an opening and closing valve provided on an exhaust passage on the side of the anode is opened (i.e., “hydrogen purge ”) and gas including unused gas and the impurities is supplied to a sub fuel cell stack provided at a downstream position, when it is determined that a power generation ability of the fuel cell stack decreases. This technique is disclosed in Japanese Patent Application Laid-open under No. 2003-77506. In addition, there is known such a technique that impurities in a supply passage in a fuel cell stack, to which fuel gas is supplied, is discharged outside by opening a cutoff valve provided at an exhaust opening, when it is determined that an output voltage of the fuel cell stack decreases. This technique is disclosed in Japanese Patent Application Laid-open under No. 9-312167. Further, there is known such a technique that, in a fuel cell system having plural supply openings and exhaust openings, fuel is supplied from the plural supply openings for the purpose of diffusing the fuel gas on an inside surface of a fuel cell system. This technique is disclosed in Japanese Patent Application Laid-open under No. 11-144753.
However, in the above-mentioned fuel cell systems, since the impurities accumulate at one portion in the fuel cell stack, the power generation efficiency in the entire fuel cell stack sometimes deteriorates. Further, in order to prevent such deterioration of the power generation efficiency, a hydrogen purge amount should be sometimes increased.